Breast cancer is considered a genetically heterogeneous and biologically diverse disease. The long-recognized clinical and phenotypic differences have been shown to correlate with differences in gene expression. Previous studies of breast tumors have identified five distinct subtypes of breast carcinomas that are associated with different clinical outcomes: luminal A (estrogen receptor (ER)+); luminal B (ER+); HER2 overexpressing; normal breast-like; and basal-like. See, Perou et al. Nature, 406(6797):747-52 (2000); Sorlie et al. PNAS, 98(19):10869-74 (2001).
Analysis of breast cancer biopsy and surgical specimens typically includes an assessment of nuclear and cell surface receptors (ER, PgR, and HER2), gene amplification of HER2 (if HER2 analysis by immunohistochemistry (IHC) is not definitive), and other prognostic tests such as microvessel invasion and proliferation markers. Endocrine therapies that target ER signaling pathways for ER+ disease and HER2-targeted therapies for HER2+ disease play a critical role in the treatment of most patients with breast cancer. However, little progress has been made in identifying effective targeted therapies for patients whose disease lacks these receptors, i.e., the so-called “triple negative” breast cancers or “TNBC”, and nonselective cytotoxic chemotherapy remains the primary therapeutic option.
The androgen receptor (AR) is the most commonly expressed nuclear hormone receptor in breast cancer, though its functional role in initiating or driving malignancy is not yet well understood. In a study of 3093 breast cancers, AR expression (10% or more nuclear staining by IHC) was observed in 77% of invasive breast tumors and across all molecular phenotypes (Collins et al., Mod Pathol 2011; 24(7):924-931). However, androgen receptor levels are not routinely assessed, since they have not been shown to predict responses to currently used therapies.
The use of AR inhibitors has been proposed as part of a therapeutic regimen for the treatment of breast cancer. See, e.g., Garay and Park, Am. J. Cancer Res. 2012; 2(4):434-445. Interest has been generated recently in the treatment of TNBC. Lack of expression of all three of estrogen receptor, progesterone receptor and HER2 predicts non-response to available endocrine (tamoxifen, aromatase inhibitors) and anti-HER2 (trastuzumab) targeted therapies. From 10 to 35% of such TNBC tumors express androgen receptor (Ogawa et al., Int J. Clin, Oncol. 2008; 13:431435). AR-targeted therapies may prove to be a valuable treatment for a large proportion of breast cancers, including triple negative cancers.
Despite the interest in androgen receptor signaling inhibition as a modality for the treatment of breast cancer, and in the treatment of TNBC in particular, there remains a need for predicting whether the individual patient will be responsive in advance of therapy. A test to predict the likelihood of whether or not a particular patient will respond to a therapy that inhibits androgen receptor signaling, and TNBC patients in particular, would be a valuable tool in planning patient treatment.